The demand for smaller and low-cost electronics has given rise to new development of fine line and high yield processes in substrate technology. Chip-on-film (COF) packaging constitutes a substantial technology to cope with the future demands of higher function, lower power consumption and miniaturization; in particular, high resolution and increasing I/O count of touch integrated circuit (IC) and display-drive IC integrated modules (TDDI) requiring extremely fine pitch COF packages. Conventionally, a flexible circuit is fabricated in a subtractive method where the copper trace pattern is formed by etching. However, this subtractive method has an inherent problem in sidewall geometry control. In a conventional semi-additive process (SAP), usually 2-3 μm thick Cu with Ni/Cr as a seed layer are used. During the removal of these layers, the wet etching process that is isotropic and not well-controlled causes simultaneous etching of both copper and the seed layer. This creates a major process limitation “undercutting,” which in turn creates several challenges for fine-line and precision patterning and leads to failure of weakened fine traces.
During diffusion bonding of the flip chip assembly, a number of independent aspects require consideration. The deformable layer must provide the requisite electrical properties with good trace integrity. It must be able to withstand sufficient pressure during contact and hence there must be enough top width on the trace so that a full contact interface with proper creep deformation and void elimination on the bonding zone is achieved. As the bond pitch reduces, the semi-additive and subtractive methods have limitations to maintain the top to bottom width (T/B) ratio as close to 1 while achieving a reasonable yield.
An alternative method is a full additive process (FAP), in which the copper pattern can be formed by electroless plating. Prior to electrochemical deposition, a thin seed layer of electroless Ni—P is formed on the polyimide (PI) that has undergone alkaline surface modification. The PI which is comprised of an imide ring can be easily opened by the incoming nucleophilic hydroxide ion forming polyamic acid salt (PAA). Since the carboxylate group on this polyamic acid is an ion exchange group, it can be reduced to deposit a Pd catalyst when treated in an aqueous Pd (II) ion solution. Once the catalyst is deposited, subsequent electroless plating is then possible. However, such a method suffers from peel strength degradation of the PI film after heat treatment and therefore is not reliable for practical applications.
U.S. Pat. Nos. 9,089,062 (Janssen) and 9,324,733 (Rogers et al) disclose methods involving polyamic acid and alkaline plating baths.